地址：http://www.w3.org/Addressing/rfc1808.txt
RFC 1808 Relative Uniform Resource Locators June 1995
格式： :///;?#
each of which, except , may be absent from a particular URL.
These components are defined as follows (a complete BNF is provided
in Section 2.2):
scheme ":" ::= scheme name, as per Section 2.1 of RFC 1738 [2].
"//" net_loc ::= network location and login information, as per
Section 3.1 of RFC 1738 [2].
"/" path ::= URL path, as per Section 3.1 of RFC 1738 [2].
";" params ::= object parameters (e.g., ";type=a" as in
Section 3.2.2 of RFC 1738 [2]).
"?" query ::= query information, as per Section 3.3 of
RFC 1738 [2].
"#" fragment ::= fragment identifier.